1. Field of the Invention
This invention relates to a method for removing impurities of a semiconductor wafer such as a silicon wafer or a SOI wafer, and to a semiconductor wafer assembly which is made during the removing process of the impurities of the semiconductor wafer. This invention also relates to a semiconductor device utilizing the semiconductor wafer cleaned in impurities by the above removing method.
2. Description of the Related Art
In a semiconductor device made of a semiconductor wafer and various semiconductor components formed on the semiconductor wafer, it was important problem to remove heavy metal impurities of a semiconductor device during the fabricating process because the performance of the semiconductor device is deteriorated if the heavy metal impurities are incorporated in the semiconductor device even in minute amount.
If the heavy metal impurities are incorporated in the semiconductor device to deteriorate the performance thereof, the product yield of IC circuit or LSI circuit composed of many integrated semiconductor devices is decreased conspicuously and thus, the IC circuits or the LSI circuits can not be fabricated profitably.
With the semiconductor device, the fundamental performance depends on the condition of p-n junction thereof. For example, with the I–V characteristic of the semiconductor device, as illustrated in the solid line of FIG. 1, when positive voltage is applied to the p-n junction, the current “I” is increased remarkably, and when negative voltage is applied to the p-n junction, the current I becomes very small up to almost zero. The illustrated characteristic is generally called as a “rectification of p-n junction”.
If there is a small amount of harmful heavy metal impurity in the vicinity of the p-n junction of the semiconductor device, as illustrated in the broken line of FIG. 1, when negative voltage is applied to the p-n junction, the current I is increased inversely (that is, a negative current is generated). The negative current is originated from a leak current in the p-n junction due to the heavy metal impurities. If the leak current is increased, the normal rectification of the p-n junction can not be realized, so that semiconductor devices with such p-n junctions as mentioned above are ranked as defective ones.
In order not to deteriorate the fundamental performance of the semiconductor device, it is required to remove the heavy metal impurities from a surface of a semiconductor wafer constructing the semiconductor device before the fabricating process of the semiconductor device or during the fabricating process. The removing operation of heavy metal impurity is called as a “Gettering treatment”.
For example, as illustrated in FIG. 2, a polycrystalline silicon film 5 is formed on the rear surface of a semiconductor wafer 3 so that harmful impurities of the surface region 6 on which semiconductor devices are fabricated are removed by the polycrystalline silicon film 5 and the nearby silicon layer. In addition, as illustrated in FIG. 3, minute defects 8 such as precipitated oxides and the related products are created so that the heavy metal impurities are absorbed by the minute defects 8. The latter operation is called as an “Intrinsic Gettering treatment”.
The above-mentioned conventional removing methods illustrated in FIGS. 2 and 3 are available in a conventional semiconductor wafer fabricating process and a conventional semiconductor device fabricating process utilizing the semiconductor wafer.
With a recent state-of-the-art LSI device, a SOI (Silicon-On-Insulator) wafer is practically employed as a semiconductor wafer so as to realize the high density integration, high speed and low electric power consumption performance thereof. As illustrated partially in FIG. 4, the SOI wafer is a wafer 13 made of a silicon substrate 10 and a thin silicon layer 12 with a thickness of 1 μm or below formed on the silicon substrate 10 via an insulator 11. Various semiconductor components are provided on the thin silicon layer 12 to fabricate the intended semiconductor device. Since the insulator 11 is generally made of oxide and embedded in the wafer, it is called a “BOX (Buried Oxide) layer”.
With the SOI wafer with the BOX layer, it is difficult to remove the harmful heavy metal impurities of the thin silicon layer 12 due to the BOX layer 11 because the diffusion velocities of the heavy metal impurities such as iron and nickel in an oxide film are very small and thus, the heavy metal impurities can not be removed through the BOX layer as an oxide layer by the Gettering treatment. Next, the impediment process of the BOX layer will be described with reference to FIGS. 5 and 6.
As illustrated in FIGS. 5 and 6, when a polycrystalline silicon film 15 is formed on the rear surface of a semiconductor wafer 14 or minute defects 21 are created to remove the heavy metal impurities of the surface regions of SOI silicon layers 17 and 22, the heavy metal impurities can not be passed through the BOX layers 18 and 23. Therefore, the heavy metal impurities can not be removed by the polycrystalline silicon layer 15 or the minute defects 21 on the conventional removing technique.